It is well known that styrene/butadiene block copolymers (SBS−s) are the earliest commericially produced styrenic monomer-based thermoplastic elastomers. SBS's can be used, for example, in the preparation of soles, adhesives and elastomeric commodities, and for modifying bitumen and plastics. The largest application field of SBS's is the production of soles, and SBS's used for soles comprise more than 50% of the world SBS's consumption.
Conventional SBS's are phase-separated block copolymers, and shearing viscosities of their melts are quite insensitive to temperature and shearing rate. In the processing of SBS's, the physical entanglement of styrene will result in the inhomogenity of viscosity of their melts (PB segments have a lower viscosity, and the interfaces of the PB and PS phases have a higher viscosity), so that AC as a flowing agent is dispersed unevenly, leading to uneven cellules. Hence, articles obtained by foaming SBS's do not meet the requirements to soles in tearing strength, stretching strength, wearing resistance, etc. In order to improve the properties of a SBS foamed article, such as stretching strength, tearing strength, chemical solvent resitance, wearing resistance and the like, an approach is to add a crosslinking agent during the foaming, but this approach suffers from two difficulties that are difficult to be overcome: one relates to the addition of the crosslinking agent—typical crosslinking agents are decomposed at a temperature below 140° C., while the plasticizing temperature of SBS's is above 170° C.; the other is that the polybutadiene phase of the conventional SBS's has a high double-bond density so that a “scorching” (a very quick chain exothermal reaction) phenomenon may appear under the action of the crosslinking agent, and the production can hardly proceed or has safety troubles or give less eligible products. Since chemically crosslinking foaming, let alone injection foaming, of SBS's cannot be achieved, the soles prepared from SBS's have high densities and inferior wearing resistance, and are therefore replaced gradually with other materials such as EVA, polyurethane (PU), etc.
Random styrenic monomer-diolefin copolymer and preparation thereof are also disclosed in numerous scientific literatures and patents. Chinese Patent Application CN 101113188A discloses a continuous method for preparing a conjugated diene/vinylarene random copolymer. The vinylarene may be styrene, the conjugated diene may be 1,3-butadiene, and the copolymer has a molecular weight ranging from 200,000 to 800,000, and a polydispersity index Mw/Mn=1.6 to 2.5. This copolymer is designed for automobile tire tread. The large molecular weight of this copolymer renders its melt viscosity too large to be foamed, let alone injection processed.
U.S. Pat. No. 4,367,325 disloses a styrene/butadiene random copolymer and a process for the production thereof. This styrene/butadiene random copolymer has a styrene monomer content of 3 to 30%, and a content of 1,2-structure in the butadiene monomer units ranging from 70 to 90%. This copolymer can be used to produce automobile tire, and has a low rolling resistance and a high wet skid resistance. This copolymer has a high melt viscosity and thus cannot be injection processed.
Furthermore, conventional styrene/butadiene random copolymers cannot be reprocessed after having been chemically crosslinked.
The currently used EVA foamed articles exhibit predominantly plastic character in stretching strength, tearing strength, compression set, wearing resistance, skid resistance, etc., and thus cannot fully meet the requirements applied by shoe production. When used as sole raw materials, polyurethanes also have drawbacks, such as a high production cost, a large toxicity of the monomers for the polyurethanes, a complexity of the foaming process, an inferior wet skid resistance, easiness of cracking, fracture of shoe heel, etc., and cannot also be injection foamed.
Thus, it is desired to provide a novel injection-foamable styrenic monomer-diolefin copolymer useful, for example, in the application of soles.